Theoretical speculation on the evolutionary adaptiveness of fears has continued from Darwin's time to the present, yet little empirical research with humans has tested hypotheses about the heritability of fear and phobias. The present research is designed to start such an empirical investigation. A self-report questionnaire on fears and phobias will be given to approximately 1200 pairs of twins ascertained through the Institute of Psychiatry in London, England. The same questionnaire has already been given to 654 individuals, comprising 151 families, throughout Minnesota, and it is also being administered to identical twins reared apart being studied at the Department of Psychology at the University of Minnesota, Minneapolis. Since the preliminary analyses done thus far on the Minnesota families indicate that fears do run in families, the focus of the current research will be on heritabilities of these fears and specificity of these fears in twins. That is, the research will examine the extent to which twin pairs share the same fears vs. similar fears through multivariate analysis of fear scores. The data collected shall also be linked with data previously collected on the Eysenck Personality Questionnaire and the Leyton Obsessional Inventory. Multivariate analysis and path analysis will also be used to determine the extent to which personality and temperament influences fears in families. Because twins are the subject of this research, the results will also suggest whether a heritable influence is operating on the etiology of fears and phobias. The prevalence of different types of phobias among English twins will be compared with the prevalences already ascertained in the sample in Minnesota to detect any gross differences in cross-cultural prevalences of phobias, a topic that has never, to our knowledge, been explored.